transition

Represents a configuration transition across a dependency edge. For example, if //package:foo depends on //package:bar with a configuration transition, then the configuration of //package:bar (and its dependencies) will be //package:foo’s configuration plus the changes specified by the transition function.

Members

• transition
• and_then

transition

transition transition(*, implementation, inputs, outputs)

A transition that reads a set of input build settings and writes a set of output build settings.

Example:

def _transition_impl(settings, attr):
    # This transition just reads the current CPU value as a demonstration.
    # A real transition could incorporate this into its followup logic.
    current_cpu = settings["//command_line_option:cpu"]
    return {"//command_line_option:compilation_mode": "dbg"}

build_in_debug_mode = transition(
    implementation = _transition_impl,
    inputs = ["//command_line_option:cpu"],
    outputs = ["//command_line_option:compilation_mode"],
)

For more details see here.

Parameters

Parameter	Description
implementation	callable; required The function implementing this transition. This function always has two parameters: settings and attr. The settings param is a dictionary whose set of keys is defined by the inputs parameter. So, for each build setting --//foo=bar, if inputs contains //foo, settings will have an entry settings['//foo']='bar'. The attr param is a reference to ctx.attr. This gives the implementation function access to the rule’s attributes to make attribute-parameterized transitions possible. This function must return a dict from build setting identifier to build setting value; this represents the configuration transition: for each entry in the returned dict, the transition updates that setting to the new value. All other settings are unchanged. This function can also return a list of dicts or a dict of dicts in the case of a split transition.
inputs	sequence of strings; required List of build settings that can be read by this transition. This becomes the key set of the settings parameter of the implementation function parameter.
outputs	sequence of strings; required List of build settings that can be written by this transition. This must be a superset of the key set of the dictionary returned by this transition.

and_then

transition transition.and_then(transition)

Returns a new transition that applies this transition followed by the given one. The second transition reads the build settings produced by this one; the original transitions are left unchanged. The result is itself a transition and may be composed further.

A composition may be used as a rule or attribute transition wherever its component transitions could be used. At most one of the composed transitions may target the exec configuration (e.g. config.exec). When two transitions in the chain split the configuration, the result has the cross product of their splits; the key for each combined split is the comma-separated concatenation of the component keys.

Parameters

Parameter	Description
transition	transition; required The transition to apply after this one.